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Side-chain engineering of medium bandgap polymer donors for efficient polymer solar cells
Organic Electronics 78 (2020) 105603
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Side-chain engineering of medium bandgap polymer donors for efficient polymer solar cells Fei Pan a, b, 1, Chenkai Sun a, b, 1, Haijun Bin a, **, Indunil Angunawela c, Wenbin Lai a, b, Lei Meng a, Harald Ade c, ***, Yongfang Li a, b, d, * a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China School of Chemical Science, University of Chinese Academy of Sciences, Beijing, 100049, China Department of Physics and Organic and Carbon Electronics Lab, North Carolina State University, Raleigh, NC, 27695, USA d Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu, 215123, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer solar cells Conjugated polymer donors Side chain engineering
Polymer solar cells (PSCs) have received widespread attentions recently due to the significant innovations of narrow bandgap n-type organic semiconductor (n-OS) acceptors. To obtain efficient PSCs, it is crucial to employ suitable donor/acceptor pair with matched electronic energy levels, complementary absorption spectra, appropriate molecular self-assembly behavior and preferred blend film morphology, which can be achieved by rational molecular structure optimization. Here we develop three D-A copolymer donors J55, J65 and J75 based on identical building blocks of bithienyl-benzodithiophene (BDTT) D-unit and bifluorine substituted benzo triazole A-unit with different flexible side-chains on BDTT unit to regulate the molecular electronic energy levels and molecular aggregation features, for further improving photovoltaic performance of the PSCs. The three D-A copolymers showed similar absorption profiles due to the identical building blocks. In comparison with the alkyl side-chain substituted polymer J55, the polymers J65 and J75 with alkylthio side-chain and alkylsilyl side-chain showed gradually down-shifted highest occupied molecular orbital energy levels (EHOMO) of 5.38 and 5.43 eV, respectively, which is beneficial for obtaining high open-circuit voltage (Voc). The favorable morphology with preferred face-on orientation and stronger integrated intensity of the π-π stacking peak was formed in J75 blend, which contributes to charge transport, thus enhancing the fill factor (FF) and Jsc. The PSC with J75 as donor and ITIC as acceptor exhibits an efficient PCE of 11.07%, with a Voc of 0.94 V, an enhanced Jsc of 16.99 mA cm 2 and a high FF of 69.29%.
1. Introduction Polymer solar cells (PSCs), employing a p-type conjugated polymer as electron donor and an n-type organic semiconductor (n-OS) as elec tron acceptor, is considered to be a promising candidate for commercial applications as renewable photovoltaic technology because of its ad vantages of low cost solution processing, light weight and flexibility for wearable devices in comparison with the traditional inorganic solar cells. In last few years, much efforts have been devoted to develop
efficient photovoltaic materials (including donors and acceptors) [1–15], efficient electrode buffer layer materials [16–20], blend morphology optimization and device engineering [6,21–25]. As a result, power conversion efficiency (PCE) of the single-junction PSCs has boosted to over 16% recently [26–28]. The PCE of PSCs is proportional to the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) of the devices. Voc is generally determined by the energy offsets between the highest occu pied molecular orbital energy level (EHOMO) of donor and the lowest
* Corresponding author. Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (H. Bin), [email protected] (H. Ade), [email protected] (Y. Li). 1 The first two authors contributed equally to this work. https://doi.org/10.1016/j.orgel.2019.105603 Received 10 October 2019; Received in revised form 21 December 2019; Accepted 21 December 2019 Available online 24 December 2019 1566-1199/© 2019 Published by Elsevier B.V.
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unoccupied molecular orbital energy level (ELUMO) of acceptor [29,30], the valid method to increase Voc are lower the EHOMO of donor and/or lift the ELUMO of acceptor. Jsc is related to the absorption spectrum of donor/acceptor (D/A) blend and the excitons dissociation efficiency at the D/A interface, thus the effective method to obtain large Jsc is to employ the absorption complementary D/A combination with preferred morphology for efficient excitons dissociation. The value of FF commonly depends on the charge carrier (both electron and hole) transport and vertical device resistance, and it could be elevated by optimizing the molecular aggregation behavior and increasing hole and electron mobilities in the blend films by rational molecular structure optimization and device engineering [19,31]. Taking these parameters into consideration, therefore it is paramount to develop the donor and acceptor photovoltaic materials with matched energy level, comple mentary absorption spectra, appropriate molecular self-assembly behavior and the preferred active layer morphology with nanoscale D/A interpenetrating networks to gain efficient PSCs. In the progress of PSCs, side-chain engineering is proved to be an efficient method to regulate the electronic energy levels, charge transfer capability and aggregation behavior, thus regulate the blend films morphology and photovoltaic performance of PSCs [3–5,32–37]. For example, based on benzodithiophene [1,2] and benzotriazole [38–40] unit, which are the classic electron donor (D) unit and electron acceptor (A) unit of organic semiconductor molecules, our group downshifted EHOMO of the 2D-conjugated polymer donors based on the bithienyl-benzodithiophene (BDTT) unit and bifluorine substituted benzotriazole (FTAZ) unit by introducing flexible alkylthio side-chain (J61) [3] or alkylsilyl side-chain (J71) [4] with the σ*(S/Si)-π*(C) bond interaction, which increased Voc and improved the photovoltaic performance of the PSCs with the polymers as donor. Moreover, we synthesized five J71 derivatives with the size of alkylsilyl side-chain on their BDTT unit and studied the effect of side-chain structure on mo lecular packing and photovoltaic properties [32]. In order to further understand the effect of side-chain substituents on the photovoltaic properties including the electronic energy levels, ab sorption spectrum, charge transfer and aggregation morphology of the polymer donors, here we synthesized three D-A copolymer donors J55, J65 and J75 based on BDTT D-unit and alkyl substituted FTAZ A-unit with a 1,1,1,3,5,5,5-heptamethyltrisiloxane terminal group on the alkyl side-chain of FTAZ unit, with different substituent on the thiophene
conjugated side chain of BDTT unit: branched alkyl for J55, branched alkylthio for J65 and branched alkylsilyl substituents for J75. The tri azole moiety of the FTAZ A-unit attaches an alkyl side-chain with siloxane terminal group which is beneficial to face-on orientation of copolymer and induces a favorable blend morphology, so that facilitate well-balanced charge carrier mobility [41]. Among the three polymers, J65 with branched alkylthio and J75 with branched alkylsilyl sub stituents possess the down-shifted EHOMO of 5.38 eV and 5.43 eV respectively, compared with that ( 5.26 eV) of J55 with branched alkyl substituent. As for the molecular aggregation, J75 shows more face-on orientation, which could be beneficial to charge carrier transport. While J55 and J65 show more edge-on orientation. The J75-based PSCs demonstrate a better photovoltaic performance with PCE of 11.07%, a Voc of 0.94 V, a Jsc of 16.99 mA cm 2 and a FF of 69.29% among the three polymer donors. The results indicate that side-chain engineering is an effective approach in designing high-performance organic photo active materials. 2. Results and discussion 2.1. Material synthesis and characterization The synthetic routes of the three polymer donors J55, J65 and J75 are described in Scheme 1, and the chemical structures of the polymer donors and ITIC acceptor are depicted in Scheme 2. The monomers and copolymers of J55, J65 and J75 were synthesized according to the procedure in previous publication [2–4,41], and the detailed synthetic procedures were described in the supporting information (SI). Molecular weights of the copolymers were measured by gel permeation chroma tography (GPC), as listed in Table 1. The number-average molecular weights (Mn) of J55, J65 and J75 are 7.9, 4.6 and 6.3 kDa with corre sponding polydispersity index (PDI) of 1.33, 1.19 and 1.34, respectively. The copolymers could be readily dissolved in common organic solvents, such as chloroform, 1-chlorobenzene, and 1,2-dichlorobenzene at room temperature. Thermal stability of the polymers was measured by thermogravi metric analysis (TGA), as showed in Fig. S1a in SI. The decomposition temperatures (Td) at 5% weight-loss are 446 � C, 350 � C and 451 � C for J55, J65 and J75, respectively, indicating that the thermal stability of the polymers is high enough for the application in PSCs. Differential
Scheme 1. Synthesis routes of monomer M1 and the polymers. 2
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Organic Electronics 78 (2020) 105603
Scheme 2. Chemical structures of copolymers and ITIC acceptor. Table 1 Molecular weights, thermal stability and physicochemical properties of polymers. Polymers
Mn (kDa)
PDI (Mw/Mn)
Td (oC)a
λmax (nm)b
λedge (nm)c
Eoptg (eV)d
J55 J65 J75
7.9 4.6 6.3
1.33 1.19 1.34
446 350 451
547, 599 545, 595 539, 584
640 640 634
1.94 1.94 1.96
a b c d e
EHOMO (eV)e 5.26 5.38 5.43
ELUMO (eV)e 3.58 3.60 3.58
5% weight loss measured by TGA under nitrogen. Polymer films on quartz plate cast from chloroform solution. Absorption edge of the polymer films. Calculated from the absorption edge of the polymer films: Eoptg ¼ 1240/λedge. Calculated according to the equation: EHOMO/LUMO ¼ -e (Eox/red þ 4.36) (eV).
scanning calorimetry (DSC) was measured for understanding crystalline behavior of the polymer donors. No obvious exothermic and endo thermic peaks were observed for the three polymer donors, indicating their amorphous characteristic. Fig. 1a shows the UV–vis absorption spectra of the polymer donors and ITIC acceptor (for comparison) films. The apparent absorption peaks with vibronic shoulder in the wavelength range from 538 to 598 nm for all the three polymers are ascribed to orderly aggregation in thin films, stemming from strong π-π stacking between the molecules. J55 film displays a main absorption peak at 599 nm and a weaker peak at 547 nm, with absorption edge at 640 nm (corresponding to an optical bandgap of 1.94 eV), and J65 and J75 films show similar main ab sorption peaks at 545 and 539 nm, respectively, with absorption edge at 640 nm and 634 nm (corresponding to an optical bandgap of 1.94 eV and 1.96 eV). J65 and J75 films with ca. 10 nm broader absorption band at the short wavelength region could be attributed to the alkylthio and alkylsilyl side-chain substituents, which leads to the reduction of the planarity of main backbone. Blend films of the polymer donors and ITIC acceptor exhibit complementary absorption spectrum ranging from 400 to 800 nm.
2.2. Photovoltaic properties To investigate photovoltaic performance of the polymer donors, PSCs based on J55, J65, J75 as donor and ITIC as acceptor with a traditional device structure of ITO (indium tin oxide)/PEDOT: PSS (poly (3,4-ethylenedioxythiophene): poly (styrene-sulfonate))/polymer donor: ITIC (1.3:1, w/w)/Ca/Al. The device fabrication conditions were optimized and the optimized conditions are: the donor: acceptor weight ratio of 1.3:1 with a total mixed solution concentration of 23 mg mL 1 in chloroform, by spin-coating at 3000 rpm, and thermal annealing (TA) at 170 � C for 2 min. Fig. 2 shows current density-voltage (J-V) curves of the optimized PSCs based on polymer: ITIC with or without TA, and the corresponding photovoltaic parameters are listed in Table 2. The PSCs based on J55: ITIC without TA (as-cast) show a PCE of 4.05% with a Voc of 0.82 V, Jsc of 12.47 mA cm 2 and FF of 39.59%. For the PSCs based on J65 and J75, the higher Voc of 0.88 and 0.92 V were obtained, mainly benefitted from their lower-lying EHOMO. Whereas the small Jsc of 12.47–13.22 mA cm 2 and FF as low as 38.82%–43.21% are reasons for low PCE of their devices. After thermal annealing treatment, the PSCs performance was greatly improved, the best PCE of the J55, J65 and J75 based PSCs increased to 6.03%, 6.91% and 11.07%, 3
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Fig. 1. Normalized absorption spectra of a) polymers and ITIC in thin films and b) blend films of the polymers and ITIC with weight ratio of 1.3:1, c) cyclic vol tammograms of the polymers films on a platinum electrode measured in 0.1 mol L 1 Bu4NPF6 acetonitrile solution at a scan rate of 20 mV s 1, d) Energy level diagram of the related materials used in the PSCs.
respectively. Obviously, the TA treatment increased the Jsc and FF significantly. As showed in Fig. 2a and Table 2, with TA treatment, the Jsc and FF values are increased from 12.47 mA cm 2 and 39.59% to 12.84 mA cm 2 and 58.02% for the J55 based device, from 12.93 mA cm 2 and 43.21% to 13.28 mA cm 2 and 55.99% for the J65 based device, from 13.22 mA cm 2 and 38.82% to 16.99 mA cm 2 and 69.29% for the J75 based device. Fig. 2b shows the input photon to converted current efficiency (IPCE) spectra of the optimized devices. All the three devices show a broad photo-response range from 300 to 800 nm. Particularly, the devices based on J75: ITIC show higher IPCE values over 70% in the wavelength range of 480–700 nm. The current density values integrated from the IPCE spectra under the AM 1.5G spectrum are 12.76 mA cm 2, 12.90 mA cm 2 and 16.32 mA cm 2 for the J55, J65 and J75-based PSCs respectively (see Table 2), which are in good agreement with the Jsc values obtained from J-V curves with less than 4% mismatch. Considering that the molecular weights of the polymer donors could influence their photovoltaic properties, the effect of the molecular weight of J75 on its device performance was investigated for the poly mer donor with the molecular weight from 2.4 to 70.7 kDa. Table S1 in SI lists the results of effect of the molecular weight of J75 on the photovoltaic performance of the PSCs based on J75:ITIC with thermal
annealing at 170 � C for 2 min. It can be seen that the PSC based on J75 with molecular weight of 6.3 kDa shows the best performance with PCE of 11.07% among the devices based on different molecular weight J75. The poorer photovoltaic performance of the PSCs based on the higher molecular weight J75 could be due to lower solubility of the polymer donor, which could result in poor morphology of the blend active layer. In order to further understand the effect of the TA treatment on the devices performance, the electron mobility (μe) and hole mobility (μh) of the neat materials, the blend active layers without (as-cast) and with TA treatment were measured using space charge limited current (SCLC) method with hole-only (ITO/PEDOT: PSS/polymers: ITIC/Au) and electron-only (ITO/ZnO/polymers: ITIC/Ca/Al) devices, and the fitted curves and data are showed in Fig. S2 and Table 2. The μe value of neat ITIC is 3.10 � 10 4 cm2 V 1s 1, the μh values of neat donors J55, J65, and J75 are 0.47 � 10 4 cm2 V 1s 1, 0.44 � 10 4 cm2 V 1s 1, and 0.26 � 10 4 cm2 V 1s 1, respectively. For blend films, the J55: ITIC active layer without TA displays a μh of 0.40 � 10 4 cm2 V 1s 1and μe of 0.15 � 10 4 cm2 V 1s 1 with a μe/μh of 0.38. The J65- and J75-based active layers without TA also have the low μh of 0.28 � 10 4 cm2 V 1s 1and 0.17 � 10 4 cm2 V 1s 1, and μe of 0.69 � 10 4 cm2 V 1s 1 and 1.16 � 10 4 cm2 V 1s 1, with μe/μh of 2.46 and 6.82 respectively. The poor performance of the PSCs without TA should be due to their lower and 4
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Fig. 2. Photovoltaic performance of the PSCs based on polymer donor: ITIC. a) J–V curves of the traditional structured PSCs based on polymer donor: ITIC (1.3:1, w/ w), under the illumination of AM1.5G, 100 mW cm 2. b) IPCE spectra of the corresponding PSCs. c) Jph versus Veff curves of the optimized devices. d) Light intensity dependence of Jsc.
unbalanced electron mobility (μe) and hole mobility (μh). With TA treatment at 170 � C for 2 min, the values of μe and μh values increased significantly with more balanced μe/μh. In particular, the PSCs based on J75: ITIC with TA treatment show a higher μh of 0.97 � 10 4 cm2 V 1s 1and μe of 1.43 � 10 4 cm2 V 1s 1 with a balanced μe/μh of 1.47, leading to the improved PCE of 11.07% with a higher Jsc of 16.99 mA cm 2 and enhanced FF of 69.29%. Exciton dissociation and charge collection behavior was studied through measurement of the dependence of photocurrent density (Jph) versus the effective voltage (Veff) of the optimized devices. The plots of Jph versus Veff of the optimized devices are showed in Fig. 2c, Jph is defined as Jph ¼ Jlight - Jdark, where Jlight is photocurrent density under illumination and Jdark is the current density in the dark condition, and Veff is defined as Veff ¼ V0 – Vbias, where V0 is the voltage at which Jph ¼ 0 and Vbias is the applied external voltage bias. It can be seen that Jph is approximately equal to saturated current density (Jsat) when Veff is higher than 2V due to that the recombination of charge carriers was suppressed by large internal electric field in the PSC at high Vbias. The value of Jph divided by Jsat (Jph/Jsat) could be employed to depict the charge dissociation probability (Pdis). Under the short-circuit condition, the calculated Pdis values from Fig. 2c are 89.07%, 82.97% and 98.50% for the devices based on J55, J65 and J75, respectively. The results
indicate that PSCs based on J75 have a more efficient exciton dissocia tion and charge collection in comparison to the devices based on J55 and J65. To understand the charge recombination behavior of the PSCs, we further studied the dependence of short-circuit current density (Jsc) versus light intensity (Plight). The relationship of Jsc and Plight could be modelled to the equation Jsc ∝ Pαlight, where the power law index α will approach to 1 if the bimolecular recombination in the device at shortcircuit condition could be ignored. As presented in Fig. 2d, the index α are 0.941, 0.916 and 0.960 for the J55, J65 and J75 based devices, respectively, which suggest there are less bimolecular recombination in the J75 based device compared with the J55 and J65 based devices. The results are consistent with the better photovoltaic performance of the J75 based device. 2.3. Morphological characterization The aggregation morphologies of the active layers generally deter mine the photovoltaic performance of the PSCs. We further investigate the effect of the side-chain engineering of the polymer donor on the photovoltaic performance by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As showed in Fig. S3, J55: ITIC 5
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about 20 nm was formed, which will benefit for the efficient exciton dissociation and charge transport and to the improved photovoltaic performance (especially for Jsc and FF). Grazing incident wide-angle X-ray diffraction (GIWAXS) measure ment was also performed to further investigate the morphology and molecular packing. The plots and 2D patterns of GIWAXS measurement are displayed in Fig. 3 and Table S2. According to the 2D patterns, the J55 and J65 blend films with ITIC show more edge-on orientation, while the J75: ITIC blend films show more face-on orientation, indicating that J75 is better for charge transport than J55 and J65. We found that although the out-of-plane (OOP) π-π stacking distances of the (010) peak are slightly increased from 3.57 Å for J55 and J65 to 3.65 Å for J75 (see Table S2 in SI), J75 shows a stronger integrated intensity of the π-π stacking peak in the OOP direction. The results indicate that the type of the side-chains affects the preferred aggregation orientation and π-π stacking features, which are critical to the charge carriers transport in the out-of-plane direction. For the as-cast blend films without TA treatment, OOP π-π stacking distances of the (010) peak of J55, J65 and J75 are 3.59, 3.59 and 3.65 Å, respectively. With TA treatment at 170 � C for 2 min, all the polymer components in their blends show the closer (010) π-π stacking distances (3.57, 3.57, 3.63 Å corresponding to J55, J65, J75, respectively) and stronger integrated intensity accompanied with narrower (010) π-π stacking peak in comparison to the as-cast films. The GIWAXS results are very well correlated with the device performances.
Table 2 Device performance of the PSCs based on polymers: ITIC (1.3:1, w/w) under AM1.5G 100 mW cm 2 illumination. 4
Devices
Voc (V)
Jsc (mA cm 2)
FF (%)
PCE (%)
μe (10
J55: ITICa
0.82 (0.82 � 0.005)c 0.81 (0.81 � 0.004) 0.88 (0.87 � 0.017) 0.93 (0.92 � 0.007) 0.92 (0.92 � 0.004) 0.94 (0.93 � 0.004)
12.47 (11.94 � 0.35) 12.84 (12.05 � 0.45) 12.93 (12.33 � 0.68) 13.28 (12.70 � 0.35) 13.22 (12.31 � 0.44) 16.99 (16.64 � 0.36)
39.59 (36.43 � 1.92) 58.02 (57.35 � 1.53) 43.21 (42.37 � 1.37) 55.99 (51.22 � 3.03) 38.82 (37.96 � 0.44) 69.29 (68.32 � 1.79)
4.05 (3.58 � 0.29) 6.03 (5.62 � 0.22) 4.92 (4.58 � 0.32) 6.91 (6.01 � 0.36) 4.72 (4.30 � 0.19) 11.07 (10.60 � 0.20)
0.15
0.40
0.21
0.41
0.69
0.28
0.83
0.36
1.16
0.17
1.43
0.97
J55: ITICb J65: ITICa J65: ITICb J75: ITICa J75: ITICb
cm2 V 1s 1)
μh (10
4
cm2 V 1s 1)
a
As-cast film. With thermal annealing at 170 � C for 2 min. c The values in parentheses are average values obtained from more than 15 devices. b
3. Conclusion
blend film has relatively uniform morphology and smooth surface with a root-mean-square (RMS) roughness of 1.15 nm, suggesting J55 could be well miscible with ITIC. With substitution of alkylthio and alkylsilyl side-chain, J65: ITIC and J75: ITIC blend films exhibit coarse surface with the increased RMS values of 1.50 nm and 4.56 nm, respectively. Particularly, the rough surface of J75: ITIC blend film with promoted crystalline morphology could provide a large contact area between the active layer and the electrode, which is beneficial to charge extraction and collection. TEM results are shown in Fig. S4. All the blend films both as-cast and with TA treatment show inhomogeneous and obviously fibrillary networks. After TA treatment, the J55 and J65 based blend films show oversize domain size on the order of 50 nm and 100 nm, respectively. But for the J75 based blends, an appropriate domain size
Three new J-series D-A copolymer donors with branched alkyl (J55), branched alkylthio (J65) and branched alkylsilyl (J75) thiophene con jugated side-chains were designed and synthesized, for investigating the effect of side-chains on EHOMO, aggregation morphology and photovol taic properties of the polymers. The polymers of J65 and J75 show about 10 nm broaden absorption spectrum in short wavelength range and down-shifted EHOMO in comparison with J55. After TA treatment at 170 � C for 2 min, J75 with branched alkylsilyl substituent exhibits a domi nating face-on molecular orientation, and higher integrated intensity of the π-π stacking peak in the out of plane direction than J55 and J65. The J75 based blend film possesses the higher and most balanced charge
Fig. 3. 1D line-cuts and 2D patterns of the GIWAXS measurements. a) OOP and b) IP line cuts of the GIWAXS patterns of the polymers film. GIWAXS patterns of three polymers film c), polymer: ITIC blend films d) with TA treatment. The sample names are labeled on the figures. 6
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carrier mobilities among the three polymers based blend films with ITIC acceptor, and the J75: ITIC blend film after TA treatment shows improved crystalline morphology and about 20 nm domain size, which is beneficial for exciton dissociation and charge transport in the corre sponding PSCs. The PSC based on J75: ITIC with TA treatment at 170 � C for 2 min exhibits a higher PCE of 11.07% (with a Voc of 0.94, a Jsc of 16.99 and a FF of 69.29%) which is much better than the devices based on J55: ITIC and J65:ITIC. The results indicate that the side chain en gineering of conjugated polymer donors is an effective way in opti mizing their aggregation morphology and improving their photovoltaic performance in PSCs.
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Declaration of competing interest There is no conflict of interests to declare. Acknowledgements This work is supported by National Natural Science Foundation of China (Nos. 91633301, 21734008 and 51820105003) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12030200). NCSU gratefully acknowledges the support of ONR grant N000141712204. X-ray data were acquired at beamlines 11.0.1.2 and 7.3.3 at the Advanced Light Source, which is supported by the Di rector, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.orgel.2019.105603. References [1] Y.F. Li, Molecular design of photovoltaic materials for polymer solar cells: towards suitable electronic energy levels and broad absorption, Acc. Chem. Res. 45 (2012) 723–733. [2] J. Min, Z.G. Zhang, S. Zhang, Y.F. Li, Conjugated side-chain-isolated D–A copolymers based on benzo[1,2-b:4,5-b0 ]dithiophene-alt-dithienylbenzotriazole: synthesis and photovoltaic properties, Chem. Mater. 24 (2012) 3247–3254. [3] H. Bin, Z.G. Zhang, L. Gao, S. Chen, L. Zhong, L. Xue, C. Yang, Y.F. Li, NonFullerene polymer solar cells based on alkylthio and fluorine substituted 2D-con jugated polymers reach 9.5% efficiency, J. Am. Chem. Soc. 138 (2016) 4657–4664. [4] H. Bin, L. Gao, Z.G. Zhang, Y. Yang, Y. Zhang, C. Zhang, S. Chen, L. Xue, C. Yang, M. Xiao, Y.F. Li, 11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor, Nat. Commun. 7 (2016) 13651. [5] L. Xue, Y. Yang, J. Xu, C. Zhang, H. Bin, Z.G. Zhang, B. Qiu, X. Li, C. Sun, L. Gao, J. Yao, X. Chen, Y. Yang, M. Xiao, Y.F. Li, Side-chain engineering on medium bandgap copolymers to suppress triplet formation for high-efficiency polymer solar cells, Adv. Mater. 29 (2017) 1703344. [6] C. Sun, F. Pan, H. Bin, J. Zhang, L. Xue, B. Qiu, Z. Wei, Z.G. Zhang, Y.F. Li, A low cost and high performance polymer donor material for polymer solar cells, Nat. Commun. 9 (2018) 743. [7] X. Xu, T. Yu, Z. Bi, W. Ma, Y. Li, Q. Peng, Realizing over 13% efficiency in greensolvent-processed nonfullerene organic solar cells enabled by 1,3,4-thiadiazolebased wide-bandgap copolymers, Adv. Mater. 29 (2017) 1703973. [8] S. Zhang, Y. Qin, J. Zhu, J. Hou, Over 14% efficiency in polymer solar cells enabled by a chlorinated polymer donor, Adv. Mater. 30 (2018) 1800868. [9] Y. Lin, J. Wang, Z.G. Zhang, H. Bai, Y.F. Li, D. Zhu, X. Zhan, An electron acceptor challenging fullerenes for efficient polymer solar cells, Adv. Mater. 27 (2015) 1170–1174. [10] Y. Lin, Q. He, F. Zhao, L. Huo, J. Mai, X. Lu, C.J. Su, T. Li, J. Wang, J. Zhu, Y. Sun, C. Wang, X. Zhan, A facile planar fused-ring electron acceptor for as-cast polymer solar cells with 8.71% efficiency, J. Am. Chem. Soc. 138 (2016) 2973–2976. [11] Y. Yang, Z.G. Zhang, H. Bin, S. Chen, L. Gao, L. Xue, C. Yang, Y.F. Li, Side-chain isomerization on an n-type organic semiconductor ITIC acceptor makes 11.77% high efficiency polymer solar cells, J. Am. Chem. Soc. 138 (2016) 15011–15018. [12] Q. Fan, W. Su, Y. Wang, B. Guo, Y. Jiang, X. Guo, F. Liu, T.P. Russell, M. Zhang, Y. F. Li, Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency, Sci. China Chem. 61 (2018) 531–537. [13] Q. Wu, D. Deng, J. Zhang, W. Zou, Y. Yang, Z. Wang, H. Li, R. Zhou, K. Lu, Z. Wei, Fluorination-substitution effect on all-small-molecule organic solar cells, Sci. China Chem. 62 (2019) 837–844.
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[1,4]dioxin-5-yl)-2-dodecyl-2H-benzo[1,2,3]triazole (PBEBT), Sol. Energy Mater. Sol. Cells 94 (2010) 1797–1802. [41] B. Fan, L. Ying, P. Zhu, F. Pan, F. Liu, J. Chen, F. Huang, Y. Cao, All-polymer solar cells based on a conjugated polymer containing siloxane-functionalized side-chains with efficiency over 10%, Adv. Mater. 29 (2017) 1703906.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: http://www.elsevier.com/locate/orgel
Side-chain engineering of medium bandgap polymer donors for efficient polymer solar cells Fei Pan a, b, 1, Chenkai Sun a, b, 1, Haijun Bin a, **, Indunil Angunawela c, Wenbin Lai a, b, Lei Meng a, Harald Ade c, ***, Yongfang Li a, b, d, * a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China School of Chemical Science, University of Chinese Academy of Sciences, Beijing, 100049, China Department of Physics and Organic and Carbon Electronics Lab, North Carolina State University, Raleigh, NC, 27695, USA d Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu, 215123, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer solar cells Conjugated polymer donors Side chain engineering
Polymer solar cells (PSCs) have received widespread attentions recently due to the significant innovations of narrow bandgap n-type organic semiconductor (n-OS) acceptors. To obtain efficient PSCs, it is crucial to employ suitable donor/acceptor pair with matched electronic energy levels, complementary absorption spectra, appropriate molecular self-assembly behavior and preferred blend film morphology, which can be achieved by rational molecular structure optimization. Here we develop three D-A copolymer donors J55, J65 and J75 based on identical building blocks of bithienyl-benzodithiophene (BDTT) D-unit and bifluorine substituted benzo triazole A-unit with different flexible side-chains on BDTT unit to regulate the molecular electronic energy levels and molecular aggregation features, for further improving photovoltaic performance of the PSCs. The three D-A copolymers showed similar absorption profiles due to the identical building blocks. In comparison with the alkyl side-chain substituted polymer J55, the polymers J65 and J75 with alkylthio side-chain and alkylsilyl side-chain showed gradually down-shifted highest occupied molecular orbital energy levels (EHOMO) of 5.38 and 5.43 eV, respectively, which is beneficial for obtaining high open-circuit voltage (Voc). The favorable morphology with preferred face-on orientation and stronger integrated intensity of the π-π stacking peak was formed in J75 blend, which contributes to charge transport, thus enhancing the fill factor (FF) and Jsc. The PSC with J75 as donor and ITIC as acceptor exhibits an efficient PCE of 11.07%, with a Voc of 0.94 V, an enhanced Jsc of 16.99 mA cm 2 and a high FF of 69.29%.
1. Introduction Polymer solar cells (PSCs), employing a p-type conjugated polymer as electron donor and an n-type organic semiconductor (n-OS) as elec tron acceptor, is considered to be a promising candidate for commercial applications as renewable photovoltaic technology because of its ad vantages of low cost solution processing, light weight and flexibility for wearable devices in comparison with the traditional inorganic solar cells. In last few years, much efforts have been devoted to develop
efficient photovoltaic materials (including donors and acceptors) [1–15], efficient electrode buffer layer materials [16–20], blend morphology optimization and device engineering [6,21–25]. As a result, power conversion efficiency (PCE) of the single-junction PSCs has boosted to over 16% recently [26–28]. The PCE of PSCs is proportional to the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) of the devices. Voc is generally determined by the energy offsets between the highest occu pied molecular orbital energy level (EHOMO) of donor and the lowest
* Corresponding author. Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (H. Bin), [email protected] (H. Ade), [email protected] (Y. Li). 1 The first two authors contributed equally to this work. https://doi.org/10.1016/j.orgel.2019.105603 Received 10 October 2019; Received in revised form 21 December 2019; Accepted 21 December 2019 Available online 24 December 2019 1566-1199/© 2019 Published by Elsevier B.V.
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unoccupied molecular orbital energy level (ELUMO) of acceptor [29,30], the valid method to increase Voc are lower the EHOMO of donor and/or lift the ELUMO of acceptor. Jsc is related to the absorption spectrum of donor/acceptor (D/A) blend and the excitons dissociation efficiency at the D/A interface, thus the effective method to obtain large Jsc is to employ the absorption complementary D/A combination with preferred morphology for efficient excitons dissociation. The value of FF commonly depends on the charge carrier (both electron and hole) transport and vertical device resistance, and it could be elevated by optimizing the molecular aggregation behavior and increasing hole and electron mobilities in the blend films by rational molecular structure optimization and device engineering [19,31]. Taking these parameters into consideration, therefore it is paramount to develop the donor and acceptor photovoltaic materials with matched energy level, comple mentary absorption spectra, appropriate molecular self-assembly behavior and the preferred active layer morphology with nanoscale D/A interpenetrating networks to gain efficient PSCs. In the progress of PSCs, side-chain engineering is proved to be an efficient method to regulate the electronic energy levels, charge transfer capability and aggregation behavior, thus regulate the blend films morphology and photovoltaic performance of PSCs [3–5,32–37]. For example, based on benzodithiophene [1,2] and benzotriazole [38–40] unit, which are the classic electron donor (D) unit and electron acceptor (A) unit of organic semiconductor molecules, our group downshifted EHOMO of the 2D-conjugated polymer donors based on the bithienyl-benzodithiophene (BDTT) unit and bifluorine substituted benzotriazole (FTAZ) unit by introducing flexible alkylthio side-chain (J61) [3] or alkylsilyl side-chain (J71) [4] with the σ*(S/Si)-π*(C) bond interaction, which increased Voc and improved the photovoltaic performance of the PSCs with the polymers as donor. Moreover, we synthesized five J71 derivatives with the size of alkylsilyl side-chain on their BDTT unit and studied the effect of side-chain structure on mo lecular packing and photovoltaic properties [32]. In order to further understand the effect of side-chain substituents on the photovoltaic properties including the electronic energy levels, ab sorption spectrum, charge transfer and aggregation morphology of the polymer donors, here we synthesized three D-A copolymer donors J55, J65 and J75 based on BDTT D-unit and alkyl substituted FTAZ A-unit with a 1,1,1,3,5,5,5-heptamethyltrisiloxane terminal group on the alkyl side-chain of FTAZ unit, with different substituent on the thiophene
conjugated side chain of BDTT unit: branched alkyl for J55, branched alkylthio for J65 and branched alkylsilyl substituents for J75. The tri azole moiety of the FTAZ A-unit attaches an alkyl side-chain with siloxane terminal group which is beneficial to face-on orientation of copolymer and induces a favorable blend morphology, so that facilitate well-balanced charge carrier mobility [41]. Among the three polymers, J65 with branched alkylthio and J75 with branched alkylsilyl sub stituents possess the down-shifted EHOMO of 5.38 eV and 5.43 eV respectively, compared with that ( 5.26 eV) of J55 with branched alkyl substituent. As for the molecular aggregation, J75 shows more face-on orientation, which could be beneficial to charge carrier transport. While J55 and J65 show more edge-on orientation. The J75-based PSCs demonstrate a better photovoltaic performance with PCE of 11.07%, a Voc of 0.94 V, a Jsc of 16.99 mA cm 2 and a FF of 69.29% among the three polymer donors. The results indicate that side-chain engineering is an effective approach in designing high-performance organic photo active materials. 2. Results and discussion 2.1. Material synthesis and characterization The synthetic routes of the three polymer donors J55, J65 and J75 are described in Scheme 1, and the chemical structures of the polymer donors and ITIC acceptor are depicted in Scheme 2. The monomers and copolymers of J55, J65 and J75 were synthesized according to the procedure in previous publication [2–4,41], and the detailed synthetic procedures were described in the supporting information (SI). Molecular weights of the copolymers were measured by gel permeation chroma tography (GPC), as listed in Table 1. The number-average molecular weights (Mn) of J55, J65 and J75 are 7.9, 4.6 and 6.3 kDa with corre sponding polydispersity index (PDI) of 1.33, 1.19 and 1.34, respectively. The copolymers could be readily dissolved in common organic solvents, such as chloroform, 1-chlorobenzene, and 1,2-dichlorobenzene at room temperature. Thermal stability of the polymers was measured by thermogravi metric analysis (TGA), as showed in Fig. S1a in SI. The decomposition temperatures (Td) at 5% weight-loss are 446 � C, 350 � C and 451 � C for J55, J65 and J75, respectively, indicating that the thermal stability of the polymers is high enough for the application in PSCs. Differential
Scheme 1. Synthesis routes of monomer M1 and the polymers. 2
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Scheme 2. Chemical structures of copolymers and ITIC acceptor. Table 1 Molecular weights, thermal stability and physicochemical properties of polymers. Polymers
Mn (kDa)
PDI (Mw/Mn)
Td (oC)a
λmax (nm)b
λedge (nm)c
Eoptg (eV)d
J55 J65 J75
7.9 4.6 6.3
1.33 1.19 1.34
446 350 451
547, 599 545, 595 539, 584
640 640 634
1.94 1.94 1.96
a b c d e
EHOMO (eV)e 5.26 5.38 5.43
ELUMO (eV)e 3.58 3.60 3.58
5% weight loss measured by TGA under nitrogen. Polymer films on quartz plate cast from chloroform solution. Absorption edge of the polymer films. Calculated from the absorption edge of the polymer films: Eoptg ¼ 1240/λedge. Calculated according to the equation: EHOMO/LUMO ¼ -e (Eox/red þ 4.36) (eV).
scanning calorimetry (DSC) was measured for understanding crystalline behavior of the polymer donors. No obvious exothermic and endo thermic peaks were observed for the three polymer donors, indicating their amorphous characteristic. Fig. 1a shows the UV–vis absorption spectra of the polymer donors and ITIC acceptor (for comparison) films. The apparent absorption peaks with vibronic shoulder in the wavelength range from 538 to 598 nm for all the three polymers are ascribed to orderly aggregation in thin films, stemming from strong π-π stacking between the molecules. J55 film displays a main absorption peak at 599 nm and a weaker peak at 547 nm, with absorption edge at 640 nm (corresponding to an optical bandgap of 1.94 eV), and J65 and J75 films show similar main ab sorption peaks at 545 and 539 nm, respectively, with absorption edge at 640 nm and 634 nm (corresponding to an optical bandgap of 1.94 eV and 1.96 eV). J65 and J75 films with ca. 10 nm broader absorption band at the short wavelength region could be attributed to the alkylthio and alkylsilyl side-chain substituents, which leads to the reduction of the planarity of main backbone. Blend films of the polymer donors and ITIC acceptor exhibit complementary absorption spectrum ranging from 400 to 800 nm.
2.2. Photovoltaic properties To investigate photovoltaic performance of the polymer donors, PSCs based on J55, J65, J75 as donor and ITIC as acceptor with a traditional device structure of ITO (indium tin oxide)/PEDOT: PSS (poly (3,4-ethylenedioxythiophene): poly (styrene-sulfonate))/polymer donor: ITIC (1.3:1, w/w)/Ca/Al. The device fabrication conditions were optimized and the optimized conditions are: the donor: acceptor weight ratio of 1.3:1 with a total mixed solution concentration of 23 mg mL 1 in chloroform, by spin-coating at 3000 rpm, and thermal annealing (TA) at 170 � C for 2 min. Fig. 2 shows current density-voltage (J-V) curves of the optimized PSCs based on polymer: ITIC with or without TA, and the corresponding photovoltaic parameters are listed in Table 2. The PSCs based on J55: ITIC without TA (as-cast) show a PCE of 4.05% with a Voc of 0.82 V, Jsc of 12.47 mA cm 2 and FF of 39.59%. For the PSCs based on J65 and J75, the higher Voc of 0.88 and 0.92 V were obtained, mainly benefitted from their lower-lying EHOMO. Whereas the small Jsc of 12.47–13.22 mA cm 2 and FF as low as 38.82%–43.21% are reasons for low PCE of their devices. After thermal annealing treatment, the PSCs performance was greatly improved, the best PCE of the J55, J65 and J75 based PSCs increased to 6.03%, 6.91% and 11.07%, 3
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Fig. 1. Normalized absorption spectra of a) polymers and ITIC in thin films and b) blend films of the polymers and ITIC with weight ratio of 1.3:1, c) cyclic vol tammograms of the polymers films on a platinum electrode measured in 0.1 mol L 1 Bu4NPF6 acetonitrile solution at a scan rate of 20 mV s 1, d) Energy level diagram of the related materials used in the PSCs.
respectively. Obviously, the TA treatment increased the Jsc and FF significantly. As showed in Fig. 2a and Table 2, with TA treatment, the Jsc and FF values are increased from 12.47 mA cm 2 and 39.59% to 12.84 mA cm 2 and 58.02% for the J55 based device, from 12.93 mA cm 2 and 43.21% to 13.28 mA cm 2 and 55.99% for the J65 based device, from 13.22 mA cm 2 and 38.82% to 16.99 mA cm 2 and 69.29% for the J75 based device. Fig. 2b shows the input photon to converted current efficiency (IPCE) spectra of the optimized devices. All the three devices show a broad photo-response range from 300 to 800 nm. Particularly, the devices based on J75: ITIC show higher IPCE values over 70% in the wavelength range of 480–700 nm. The current density values integrated from the IPCE spectra under the AM 1.5G spectrum are 12.76 mA cm 2, 12.90 mA cm 2 and 16.32 mA cm 2 for the J55, J65 and J75-based PSCs respectively (see Table 2), which are in good agreement with the Jsc values obtained from J-V curves with less than 4% mismatch. Considering that the molecular weights of the polymer donors could influence their photovoltaic properties, the effect of the molecular weight of J75 on its device performance was investigated for the poly mer donor with the molecular weight from 2.4 to 70.7 kDa. Table S1 in SI lists the results of effect of the molecular weight of J75 on the photovoltaic performance of the PSCs based on J75:ITIC with thermal
annealing at 170 � C for 2 min. It can be seen that the PSC based on J75 with molecular weight of 6.3 kDa shows the best performance with PCE of 11.07% among the devices based on different molecular weight J75. The poorer photovoltaic performance of the PSCs based on the higher molecular weight J75 could be due to lower solubility of the polymer donor, which could result in poor morphology of the blend active layer. In order to further understand the effect of the TA treatment on the devices performance, the electron mobility (μe) and hole mobility (μh) of the neat materials, the blend active layers without (as-cast) and with TA treatment were measured using space charge limited current (SCLC) method with hole-only (ITO/PEDOT: PSS/polymers: ITIC/Au) and electron-only (ITO/ZnO/polymers: ITIC/Ca/Al) devices, and the fitted curves and data are showed in Fig. S2 and Table 2. The μe value of neat ITIC is 3.10 � 10 4 cm2 V 1s 1, the μh values of neat donors J55, J65, and J75 are 0.47 � 10 4 cm2 V 1s 1, 0.44 � 10 4 cm2 V 1s 1, and 0.26 � 10 4 cm2 V 1s 1, respectively. For blend films, the J55: ITIC active layer without TA displays a μh of 0.40 � 10 4 cm2 V 1s 1and μe of 0.15 � 10 4 cm2 V 1s 1 with a μe/μh of 0.38. The J65- and J75-based active layers without TA also have the low μh of 0.28 � 10 4 cm2 V 1s 1and 0.17 � 10 4 cm2 V 1s 1, and μe of 0.69 � 10 4 cm2 V 1s 1 and 1.16 � 10 4 cm2 V 1s 1, with μe/μh of 2.46 and 6.82 respectively. The poor performance of the PSCs without TA should be due to their lower and 4
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Fig. 2. Photovoltaic performance of the PSCs based on polymer donor: ITIC. a) J–V curves of the traditional structured PSCs based on polymer donor: ITIC (1.3:1, w/ w), under the illumination of AM1.5G, 100 mW cm 2. b) IPCE spectra of the corresponding PSCs. c) Jph versus Veff curves of the optimized devices. d) Light intensity dependence of Jsc.
unbalanced electron mobility (μe) and hole mobility (μh). With TA treatment at 170 � C for 2 min, the values of μe and μh values increased significantly with more balanced μe/μh. In particular, the PSCs based on J75: ITIC with TA treatment show a higher μh of 0.97 � 10 4 cm2 V 1s 1and μe of 1.43 � 10 4 cm2 V 1s 1 with a balanced μe/μh of 1.47, leading to the improved PCE of 11.07% with a higher Jsc of 16.99 mA cm 2 and enhanced FF of 69.29%. Exciton dissociation and charge collection behavior was studied through measurement of the dependence of photocurrent density (Jph) versus the effective voltage (Veff) of the optimized devices. The plots of Jph versus Veff of the optimized devices are showed in Fig. 2c, Jph is defined as Jph ¼ Jlight - Jdark, where Jlight is photocurrent density under illumination and Jdark is the current density in the dark condition, and Veff is defined as Veff ¼ V0 – Vbias, where V0 is the voltage at which Jph ¼ 0 and Vbias is the applied external voltage bias. It can be seen that Jph is approximately equal to saturated current density (Jsat) when Veff is higher than 2V due to that the recombination of charge carriers was suppressed by large internal electric field in the PSC at high Vbias. The value of Jph divided by Jsat (Jph/Jsat) could be employed to depict the charge dissociation probability (Pdis). Under the short-circuit condition, the calculated Pdis values from Fig. 2c are 89.07%, 82.97% and 98.50% for the devices based on J55, J65 and J75, respectively. The results
indicate that PSCs based on J75 have a more efficient exciton dissocia tion and charge collection in comparison to the devices based on J55 and J65. To understand the charge recombination behavior of the PSCs, we further studied the dependence of short-circuit current density (Jsc) versus light intensity (Plight). The relationship of Jsc and Plight could be modelled to the equation Jsc ∝ Pαlight, where the power law index α will approach to 1 if the bimolecular recombination in the device at shortcircuit condition could be ignored. As presented in Fig. 2d, the index α are 0.941, 0.916 and 0.960 for the J55, J65 and J75 based devices, respectively, which suggest there are less bimolecular recombination in the J75 based device compared with the J55 and J65 based devices. The results are consistent with the better photovoltaic performance of the J75 based device. 2.3. Morphological characterization The aggregation morphologies of the active layers generally deter mine the photovoltaic performance of the PSCs. We further investigate the effect of the side-chain engineering of the polymer donor on the photovoltaic performance by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As showed in Fig. S3, J55: ITIC 5
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about 20 nm was formed, which will benefit for the efficient exciton dissociation and charge transport and to the improved photovoltaic performance (especially for Jsc and FF). Grazing incident wide-angle X-ray diffraction (GIWAXS) measure ment was also performed to further investigate the morphology and molecular packing. The plots and 2D patterns of GIWAXS measurement are displayed in Fig. 3 and Table S2. According to the 2D patterns, the J55 and J65 blend films with ITIC show more edge-on orientation, while the J75: ITIC blend films show more face-on orientation, indicating that J75 is better for charge transport than J55 and J65. We found that although the out-of-plane (OOP) π-π stacking distances of the (010) peak are slightly increased from 3.57 Å for J55 and J65 to 3.65 Å for J75 (see Table S2 in SI), J75 shows a stronger integrated intensity of the π-π stacking peak in the OOP direction. The results indicate that the type of the side-chains affects the preferred aggregation orientation and π-π stacking features, which are critical to the charge carriers transport in the out-of-plane direction. For the as-cast blend films without TA treatment, OOP π-π stacking distances of the (010) peak of J55, J65 and J75 are 3.59, 3.59 and 3.65 Å, respectively. With TA treatment at 170 � C for 2 min, all the polymer components in their blends show the closer (010) π-π stacking distances (3.57, 3.57, 3.63 Å corresponding to J55, J65, J75, respectively) and stronger integrated intensity accompanied with narrower (010) π-π stacking peak in comparison to the as-cast films. The GIWAXS results are very well correlated with the device performances.
Table 2 Device performance of the PSCs based on polymers: ITIC (1.3:1, w/w) under AM1.5G 100 mW cm 2 illumination. 4
Devices
Voc (V)
Jsc (mA cm 2)
FF (%)
PCE (%)
μe (10
J55: ITICa
0.82 (0.82 � 0.005)c 0.81 (0.81 � 0.004) 0.88 (0.87 � 0.017) 0.93 (0.92 � 0.007) 0.92 (0.92 � 0.004) 0.94 (0.93 � 0.004)
12.47 (11.94 � 0.35) 12.84 (12.05 � 0.45) 12.93 (12.33 � 0.68) 13.28 (12.70 � 0.35) 13.22 (12.31 � 0.44) 16.99 (16.64 � 0.36)
39.59 (36.43 � 1.92) 58.02 (57.35 � 1.53) 43.21 (42.37 � 1.37) 55.99 (51.22 � 3.03) 38.82 (37.96 � 0.44) 69.29 (68.32 � 1.79)
4.05 (3.58 � 0.29) 6.03 (5.62 � 0.22) 4.92 (4.58 � 0.32) 6.91 (6.01 � 0.36) 4.72 (4.30 � 0.19) 11.07 (10.60 � 0.20)
0.15
0.40
0.21
0.41
0.69
0.28
0.83
0.36
1.16
0.17
1.43
0.97
J55: ITICb J65: ITICa J65: ITICb J75: ITICa J75: ITICb
cm2 V 1s 1)
μh (10
4
cm2 V 1s 1)
a
As-cast film. With thermal annealing at 170 � C for 2 min. c The values in parentheses are average values obtained from more than 15 devices. b
3. Conclusion
blend film has relatively uniform morphology and smooth surface with a root-mean-square (RMS) roughness of 1.15 nm, suggesting J55 could be well miscible with ITIC. With substitution of alkylthio and alkylsilyl side-chain, J65: ITIC and J75: ITIC blend films exhibit coarse surface with the increased RMS values of 1.50 nm and 4.56 nm, respectively. Particularly, the rough surface of J75: ITIC blend film with promoted crystalline morphology could provide a large contact area between the active layer and the electrode, which is beneficial to charge extraction and collection. TEM results are shown in Fig. S4. All the blend films both as-cast and with TA treatment show inhomogeneous and obviously fibrillary networks. After TA treatment, the J55 and J65 based blend films show oversize domain size on the order of 50 nm and 100 nm, respectively. But for the J75 based blends, an appropriate domain size
Three new J-series D-A copolymer donors with branched alkyl (J55), branched alkylthio (J65) and branched alkylsilyl (J75) thiophene con jugated side-chains were designed and synthesized, for investigating the effect of side-chains on EHOMO, aggregation morphology and photovol taic properties of the polymers. The polymers of J65 and J75 show about 10 nm broaden absorption spectrum in short wavelength range and down-shifted EHOMO in comparison with J55. After TA treatment at 170 � C for 2 min, J75 with branched alkylsilyl substituent exhibits a domi nating face-on molecular orientation, and higher integrated intensity of the π-π stacking peak in the out of plane direction than J55 and J65. The J75 based blend film possesses the higher and most balanced charge
Fig. 3. 1D line-cuts and 2D patterns of the GIWAXS measurements. a) OOP and b) IP line cuts of the GIWAXS patterns of the polymers film. GIWAXS patterns of three polymers film c), polymer: ITIC blend films d) with TA treatment. The sample names are labeled on the figures. 6
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Organic Electronics 78 (2020) 105603
carrier mobilities among the three polymers based blend films with ITIC acceptor, and the J75: ITIC blend film after TA treatment shows improved crystalline morphology and about 20 nm domain size, which is beneficial for exciton dissociation and charge transport in the corre sponding PSCs. The PSC based on J75: ITIC with TA treatment at 170 � C for 2 min exhibits a higher PCE of 11.07% (with a Voc of 0.94, a Jsc of 16.99 and a FF of 69.29%) which is much better than the devices based on J55: ITIC and J65:ITIC. The results indicate that the side chain en gineering of conjugated polymer donors is an effective way in opti mizing their aggregation morphology and improving their photovoltaic performance in PSCs.
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Declaration of competing interest There is no conflict of interests to declare. Acknowledgements This work is supported by National Natural Science Foundation of China (Nos. 91633301, 21734008 and 51820105003) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12030200). NCSU gratefully acknowledges the support of ONR grant N000141712204. X-ray data were acquired at beamlines 11.0.1.2 and 7.3.3 at the Advanced Light Source, which is supported by the Di rector, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.orgel.2019.105603. References [1] Y.F. Li, Molecular design of photovoltaic materials for polymer solar cells: towards suitable electronic energy levels and broad absorption, Acc. Chem. Res. 45 (2012) 723–733. [2] J. Min, Z.G. Zhang, S. Zhang, Y.F. Li, Conjugated side-chain-isolated D–A copolymers based on benzo[1,2-b:4,5-b0 ]dithiophene-alt-dithienylbenzotriazole: synthesis and photovoltaic properties, Chem. Mater. 24 (2012) 3247–3254. [3] H. Bin, Z.G. Zhang, L. Gao, S. Chen, L. Zhong, L. Xue, C. Yang, Y.F. Li, NonFullerene polymer solar cells based on alkylthio and fluorine substituted 2D-con jugated polymers reach 9.5% efficiency, J. Am. Chem. Soc. 138 (2016) 4657–4664. [4] H. Bin, L. Gao, Z.G. Zhang, Y. Yang, Y. Zhang, C. Zhang, S. Chen, L. Xue, C. Yang, M. Xiao, Y.F. Li, 11.4% Efficiency non-fullerene polymer solar cells with trialkylsilyl substituted 2D-conjugated polymer as donor, Nat. Commun. 7 (2016) 13651. [5] L. Xue, Y. Yang, J. Xu, C. Zhang, H. Bin, Z.G. Zhang, B. Qiu, X. Li, C. Sun, L. Gao, J. Yao, X. Chen, Y. Yang, M. Xiao, Y.F. Li, Side-chain engineering on medium bandgap copolymers to suppress triplet formation for high-efficiency polymer solar cells, Adv. Mater. 29 (2017) 1703344. [6] C. Sun, F. Pan, H. Bin, J. Zhang, L. Xue, B. Qiu, Z. Wei, Z.G. Zhang, Y.F. Li, A low cost and high performance polymer donor material for polymer solar cells, Nat. Commun. 9 (2018) 743. [7] X. Xu, T. Yu, Z. Bi, W. Ma, Y. Li, Q. Peng, Realizing over 13% efficiency in greensolvent-processed nonfullerene organic solar cells enabled by 1,3,4-thiadiazolebased wide-bandgap copolymers, Adv. Mater. 29 (2017) 1703973. [8] S. Zhang, Y. Qin, J. Zhu, J. Hou, Over 14% efficiency in polymer solar cells enabled by a chlorinated polymer donor, Adv. Mater. 30 (2018) 1800868. [9] Y. Lin, J. Wang, Z.G. Zhang, H. Bai, Y.F. Li, D. Zhu, X. Zhan, An electron acceptor challenging fullerenes for efficient polymer solar cells, Adv. Mater. 27 (2015) 1170–1174. [10] Y. Lin, Q. He, F. Zhao, L. Huo, J. Mai, X. Lu, C.J. Su, T. Li, J. Wang, J. Zhu, Y. Sun, C. Wang, X. Zhan, A facile planar fused-ring electron acceptor for as-cast polymer solar cells with 8.71% efficiency, J. Am. Chem. Soc. 138 (2016) 2973–2976. [11] Y. Yang, Z.G. Zhang, H. Bin, S. Chen, L. Gao, L. Xue, C. Yang, Y.F. Li, Side-chain isomerization on an n-type organic semiconductor ITIC acceptor makes 11.77% high efficiency polymer solar cells, J. Am. Chem. Soc. 138 (2016) 15011–15018. [12] Q. Fan, W. Su, Y. Wang, B. Guo, Y. Jiang, X. Guo, F. Liu, T.P. Russell, M. Zhang, Y. F. Li, Synergistic effect of fluorination on both donor and acceptor materials for high performance non-fullerene polymer solar cells with 13.5% efficiency, Sci. China Chem. 61 (2018) 531–537. [13] Q. Wu, D. Deng, J. Zhang, W. Zou, Y. Yang, Z. Wang, H. Li, R. Zhou, K. Lu, Z. Wei, Fluorination-substitution effect on all-small-molecule organic solar cells, Sci. China Chem. 62 (2019) 837–844.
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